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    SubjectRe: [PATCH] Enforce valid shift values in clocksource_register()
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    On Thu, 2008-11-27 at 01:08 +0100, Thomas Gleixner wrote:
    > On Wed, 26 Nov 2008, John Stultz wrote:
    >
    > > Thomas noted that some of the timekeeping core assumes
    > > clocksource shift values will be less then 32. However,
    > > we do nothing to enforce such limits.
    > >
    > > This patch prints a warning if a clocksource with an invalid
    > > shift value is attempted to be registered, and will refuse to
    > > register the clocksource
    > >
    > > I could not find any clocksources that use 32 or higher for a
    > > shift value, so this should be only a preventive warning for
    > > future clocksource writers.
    > >
    > > Signed-off-by: John Stultz <johnstul@us.ibm.com>
    > >
    > > diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
    > > index 9ed2eec..4a3bb13 100644
    > > --- a/kernel/time/clocksource.c
    > > +++ b/kernel/time/clocksource.c
    > > @@ -318,7 +318,7 @@ static int clocksource_enqueue(struct clocksource *c)
    > > * clocksource_register - Used to install new clocksources
    > > * @t: clocksource to be registered
    > > *
    > > - * Returns -EBUSY if registration fails, zero otherwise.
    > > + * Returns -EBUSY or -EINVAL if registration fails, zero otherwise.
    > > */
    > > int clocksource_register(struct clocksource *c)
    > > {
    > > @@ -327,7 +327,14 @@ int clocksource_register(struct clocksource *c)
    > >
    > > /* save mult_orig on registration */
    > > c->mult_orig = c->mult;
    > > -
    > > + if (c->shift >= 32) {
    > > + printk(KERN_WARNING "===============================\n");
    > > + printk(KERN_WARNING "WARNING: Cannot register %s clocksource\n",
    > > + c->name);
    > > + printk(KERN_WARNING "The shift value must be less then 32\n");
    > > + printk(KERN_WARNING "===============================\n");
    > > + return -EINVAL;
    >
    > Just setting the shift value to 31 along with a WARN_ON() should be
    > enough. We don't need to kill the clocksource in that case, as this
    > can be nasty when it happens in the early boot code where we dont have
    > any output.

    Well, we can't tweak the shift value without changing the mult. And that
    seemed a bit overreaching (but might be safe). I'll give it a shot.

    Also early boot we should have the jiffies clocksource around, so unless
    I'm forgetting something, I don't think it has early boot concerns.

    thanks
    -john



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